Field of Invention
The present invention relates to a technical field of supercritical water, and more particularly to a system for preparing nanoparticles by supercritical hydrothermal synthesis.
Description of Related Arts
Nano-particles have special structural properties, such as large specific surface area, small size effect, interfacial effect, quantum effect and quantum tunneling effect, so that the nano-particles have different unique properties from traditional materials, and the specific electrical, thermal, magnetic, optical and mechanical properties are the most noticeable and have important application value. Traditional methods for preparing the nano-particles include the electrolysis method, the spray pyrolysis method, the gas phase reaction method, the liquid phase reaction method, the micro emulsion method, and the mechanical crushing method. In industrial production, the more mature electrolysis process and the mechanical grinding process have high production electricity consumption, low production efficiency and long production period. In recent years, the chemical liquid phase method has been the more active method for preparing nanoparticles. However, it generally needs a large number of organic solvents or highly toxic additive components, which results in serious pollution in the production, thereby greatly limiting the application. Therefore, it is important to find a green and highly effective method for preparing nanoparticles which takes water as the reaction medium.
Supercritical water (abbreviated as SCW) is the water under special conditions whose temperature and pressure are higher than the critical point (T=374.15° C. and P=22.12 MPa). The SCW has the nature of both liquid and gaseous water. In this state, only a small amount of hydrogen bonds exist in the water, the dielectric constant of the SCW close to that of the organic solvent, and the SCW has a high diffusion coefficient and a low viscosity. Supercritical hydrothermal synthesis reaction is that in a closed high-pressure reactor, the SCW is taken as the reaction medium for allowing the metal salt to hydrolyze and dehydrate in the hydrothermal medium, so as to further nucleate and grow, for finally forming the nano-grains with a certain particle size and crystal morphology. In the SCW, the non-polar gases such as reductive organic matters or H2 are able to be mixed with the SCW to form a homogeneous reaction system, so that metal oxides are able be efficiently reduced to produce high-purity metal nanoparticles. The reaction medium is the SCW, the reaction process in the closed high-pressure vessel, so that no other pollutants are introduced in the reaction process. Therefore, the supercritical hydrothermal synthesis reaction is considered to be a green environmental nano-preparation technology.
Currently, no complete process scheme for preparing nano metal materials by supercritical hydrothermal synthesis exists, which relates to some critical technical problems as follows.
(1) Achieve rapid heating of metal salt solution. Continuous supercritical hydrothermal synthesis of nanoparticles in the process, usually adopts directly mixing the metal salt solution at normal temperature and the SCW in the mixer, and quickly heating to the supercritical state. The process has the advantages of rapid heating rate, rapid generation of a large number of crystal nucleuses, short residence time, and effective inhibition of grain growth and agglomeration. In the design of the reactor, the mixing manner of two fluids is the key to determine whether the system is able to run well. In the field of engineering, the method that static mixers are adopted to mix the high-temperature and high-pressure fluid has been extensively studied, but how to control the solid particle generation reaction by controlling the mixing within a very short time period of mixing is a technical problem. Good evaluation criteria of the mixers should be able to achieve rapid and symmetrical mixing of two fluids so as to achieve fast and uniform nucleation, thereby controlling the quality of the product.
(2) Adopt organic ligands to prevent agglomeration of nanoparticles. Due to the characteristics such as a large surface area and a large surface energy, the surface of the nanoparticles is in the absence of adjacent coordination atoms, and is in the energy instability, which is prone to agglomeration. Therefore, it is necessary to add organic ligands during the hydrothermal synthesis of nanoparticles. Organic ligands are able to enhance the steric hindrance effect among the particles and inhibit particle growth and agglomeration. In general, the organic ligands used in the supercritical hydrothermal synthesis reaction are added excessively, so it is needed to recycle and reuse the residual organic ligands after reaction, for achieving organic waste emission.